Less Documented Diseases

LESS DOCUMENTED DISEASES It shouldbe emphasized that the absence ofreported diseases for a particularspecies doesnot incan that the speciesis not subjectto significantdiseases. Molluscan species undoubtedlycontract ixnportant diseases about which we know nothing. They also contract Chseasesthat arewell known but aboutwhich very little technicalinformation is available asto their cause, prevention, and management, A number of diseasesand parasites of molluscsare mentioned briefly in thetechnical literature but notin this guide,because too littleis knownabout their relevanceto molluscculture or the effectsan their host. Asxnore species ofxnolluscs are farmed and as the production requirements for commonlycultured species become more rigorous, we will learnmore about the importance Ofthese diseases, their cause,management, and prevention. It is alsoimportant to note thatdiseases which are not important to onespecies in a givenarea can be important if they areintroduced to a newhost species or evento the samehost species if it hasnot adaptedto the diseaseorganisxn. Rickettsia and Chlaxnydia of Molluscs Rickettsiaand chlamydia are intracellular bacteria that is, theylive inside cells! thatcause diseases in mammals,including xnan. Most bacteria, including those that can causedisease, do not actually reside inside living cells although they may live inside the hostorganism in various locations. Since there is noevidence that the similar organisms in molluscscause diseases of maxnmalsor man,they shouldbe referredto as rickettsia-likeor chlamydia-like. Thisgroup coxnprises some of the most coxnmanly observed xnicroorganisxns in the tissuesof bivalvemolluscs. They occur in healthyanixnals without causingany apparent detrimentaleffect, In severalinstances they have been blamed for xnassivemortalities of scallops,including the sea scallop Placopecten magellanicus!. This xnay eventually prove to betrue, but further study on these diseases is required before we fully understand their significance.Themicraorganisms, essentially bacteria that are adapted tograw inside the ceHsofthe host, are most commonly found in theepithelial tissues of the gills and digestive I,1hand of the host bivalve mollusc. Thesemicroorganisms occur in a varietyof species of bivalvemolluscs throughout the v:orjd.Chlamydia-like organisms have been reported in thebay scallap Argopecten irradians!,the Portuguese oyster Crassostrea angulata!, and the hard-shell clam Mercenaria m ercenaria!. Rickettsia-likeorganisms have been reported in the Pacificoyster Crassostrea gipi~!, the Eastern oyster Crassostrea virgini ca!, Donax trunculus, the hard-shell clam M. mercenaria!,thesoft-shell clam Mya arenaria!, the sea scallop Placopecten magellanicus!, thePacific razor claxn Siliqua patula!, the thin tellin Tellinatenuis!, the Manilaclaxn 42 / MOLLUSC DISEASES / Elston Tapesphilippinarum!, the Japanesescallop Pati nopectenyessoensis!, the Europeanflat oyster Ostrea edulis!, and the Palourde clam Ruditapes phili ppinarum!. References Comps,M. 1982. Etudemorphologique d'une infection rickettsienne de la palourdeRu- ditapes philippinarum Adam and Reeves. Revue des Travaux de l'Institut des Peches Mari ti mes 46!: 141-145. Comps,M. 1983. Infections rickettsiennes chezles mollusquesbivalves des cotes franqaises. Rapports et Proces-Verbauxdes Reunions Consei l International pour l'Explora- tion de la Mer 182:134-136. Comps,M., and R. Raimbault. 1978, Infection rickettsienne de la glande digestive de Donax trunculus Linn'. Science et Peche,Bulletin de l lnstitut des PechesMaritimes 281. Comps,M., J. P. Deltreil, and C. Vago. 1979. Un microorganismede type rickettsienne chezI'Huitre portugaise Crassostreaangulata Lmk. ComptesRendus Academic des Sciences Paris 289, Shirie D;169-171, Elston, R. A. 1986. Occurrence of branchial rickettsiales-like infections in two bivalve molluscs,Tapes japonica andPati nopecten yessoensis, with commentson their significance. Journal of Fish Diseases 9:69-71. Harshbarger,J. C., S. C. Chang,and S. V. Otto. 1977.Chlarnydiae with phages!,rnycoplas- mas, and rickettsia in Chesapeake Bay bivalves. Science 196:666-668. Meyers,T. R. 1979. Preliminary studies on a chlamydial agent in the digestive diverticular epithelium of hard clams Mercenaria mercenaria L.! from Great South Bay, New York. Journal of Fish Diseases 2.179-189. Morrison, C., and G, Shurn, 1982. Chlamydia-like organismsin the digestive diverticula of the bay scallop, Argopecten irradians Lmk!. Journal of Fish Diseases 5:173-184. Nuclear Inclusion X NIX! Nuclearinclusion X, or NIX, is a diseaseof the Pacificrazor clam, Siliqua patula. It is causedby a highly specializedand very large type of rickettsia-like microorganisill. It was first discoveredon the Pacific coast in Washingtonin 1983in associationwith a mas- sive mortality of the razor clam. It infects the gill epithelial tissues and interferes with the respiratory processesof the clam. Virtually all clams in Washington are infected, as well as some populations in Oregon and British Columbia. The diseasepersists at a lowlevel in clamsduring the winter and spring. In some LessDocumented Diseases /43 yearsthe infection can greatly increase in intensity during the summer and fall, when mortalitiesassociated with the diseaseusually occur. Refereace Klston,R.A. 1986.An intranuclear pathogen [nuclear inclusion X NIX!Jassociated with massivemortalities ofthe Pacific razor clam, Siliqua patula. Journal of Invertebrate Pathology47:93-104. MalpequeBay Disease of the American Oyster MalpequeBay disease isa widelyknown but poorly understood disease that caused severemortalities inAmerican oysters Crassostrea virginica! inMalpeque Bay in the Canadianmaritime province ofPrince Edward Island starting in1915 and continuing throughthe1930s. The geographical expansion ofthe disease, first observed a year after substantialplantings ofseed oysters imported from the United States, isconsidered evi- denceforan infectious cause ofthe disease. More than 90% oforiginal stocks were reported to have succumbed to the disease. Theoysters affected bythe disease reportedly show visceral shrinkage, a translucent quahty,reduced growth, and failure to spawn. The cause ofMalpeque Baydisease has neverbeen determined with certainty. Reference Reedier,A,W. H., and R. R. Logic. 1947, Serious mortalities inPrince Edward Island oysterscaused bya contagiousdisease, Transactions ofthe Royal Society ofCanada lit;3,5!:73-89. Gill Parasiteof theJapanese Scallop Thegill parasite, longrecognized inJapan and described inthe Japanese-language literature,wasdiscovered ina group ofscallops Patinopecten yessoensis! proposed for introductiontoNorth America. First thought tobe a parasiticbarnacle, thisorganism is iiowrecognized asan unusual form of parasitic copepod, Pectenophilus ornatus. These niisedyellow bodies on the surface ofthe gill can be as large as 8 mmin diameter see Pismire11!.In bottom culture in Japan, it was reported that up to 60 parasites can occur on «nindividual scallop. The number ofparasites was greatly reduced inhanging cultures of ;col ops.Thisparasite, undoubtedly a burden tothe scallop when it occurs inlarge number~,isof more direct aesthetic significance, sincea single parasite renders thescallop iinacceptableasa wholeanimal product. It wasalso reported tooccur on another species ofscallop inJapan, Chlamys akazara. Figure 11. Japanese scallop infected with a single specimen of what is believed to be an u.nusual parasitic copepod, Pectenophi tus omatus, shown at the arrow. As many as 60 of these parasites can occur on the gills of one scallop, but a single parasite renders the scallop unmarketable as a whole animal product, From Elston et al. 1985! References Gulka, G., P. W. Chang, and K. A. Marti. 1983. Prokaryotic infection associated with a mass mortality of the sea scallop Placopecten magelianicus. Journal of Fish Diseases 6:355- 364. Nagawawa, K., J. Bresciani, and J. Liitzen. 1988. Morphology of Pectenaphilus ornatu8, new genus, new species, a copepod parasite of the Japanese scallop Patinopecten yessoensis.,:.::,.', Journal of Crustacean Biology 8;81.-42. LessDontrnented Diseases / 45 Miscellaneous Diseases Mytilicolaorientalis isa parasite foundin the digestive tractof molluscs including severalspecies ofoysters, mussels, andslipper shells. Ithas been reported inPacific oysters,Crassostrea gigas,inCalifornia, Oregon,and Washington. Thisparasite was introducedintoFrance withimported Pacific oysters andis now present inthe Arcachon regionofFrance. Mytilicola intestinalis isa closely relatedspecies foundin Europe. Theparasite cancause damage tothe lining ofthe digestive tractwhere it attaches tothe host. InEurope, itis reported thatmortalities ofmussels werecaused byM. r'ntesti- naliswhen infestation reachedlevels of 5-10 parasites permussel. Substantial mortalities associatedwiththis parasite havenot been reported inNorth American speciesof molluscs; however,Severalinfestations viruses, caninloweraddition the tocondition those previouslyindex of oysters.discussed, havebeen observed inthe tissuesofbivalve molluscs. These include a herpes-like virusinthe American oyster, Crassostreavirginica,in the state ofMaine. Preliminary studiessuggested thatvirus was associatedwithmortalities atelevated seawater temperatures 8'C-30'Cas compared with 18'C-20'C!,butfurther studies would berequired toprove that the virus caused theoyster deaths,Viruses similar tothat causing velarvirus disease inlarval Pacific oysters, Crassos- treagigas, havebeen found inthe blood cellsand

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